1. Field of the Invention
The present invention pertains generally to repairing intervertebral disc disorders, and more particularly to an implant and surgical procedure for repairing a degenerated intervertebral disc.
2. Description of the Background Art
An estimated 4.1 million Americans annually report intervertebral disc disorders, with a significant portion of them adding to the nearly 5.2 million low-back disabled. Though the origin of low-back pain is varied, the intervertebral disc is thought to be a primary source in many cases, and is an initiating factor in others where a degenerated disc has led to altered spinal mechanics and non-physiologic stress in surrounding tissues.
The intervertebral disc is a complex structure consisting of three distinct parts: the nucleus pulposus; the annulus fibrosus; and the cartilaginous end-plates. The nucleus pulposus is a viscous, mucoprotein gel that is approximately centrally located within the disc. It consists of abundant sulfated glycosaxninoglycans in a loose network of type II collagen, with a water content that is highest at birth (approximately 80%) and decreases with age. The annulus fibrosus is that portion of the disc which becomes differentiated from the periphery of the nucleus and forms the outer boundary of the disc. The transition between the nucleus and the annulus is progressively more indefinite with age. The annulus is made up of coarse type I collagen fibers oriented obliquely and arranged in lamellae which attach the adjacent vertebral bodies. The fibers run the same direction within a given lamella but opposite to those in adjacent lamellae. The collagen content of the disc steadily increases from the center of the nucleus to the outer layers of the annulus, where collagen reaches 70% or more of the dry weight. Type I and II collagen are distributed radially in opposing concentration gradients. The cartilaginous end-plates cover the end surfaces of the vertebral bodies and serve as the cranial and caudal surfaces of the intervertebral disc. They are composed predominately of hyaline cartilage.
The disc derives its structural properties largely through its ability to attract and retain water. The proteoglycans of the nucleus attract water osmotically, exerting a swelling pressure that enables the disc to support spinal compressive loads. The pressurized nucleus also creates tensile pre-stress within the annulus and ligamentous structures surrounding the disc. In other words, although the disc principally supports compressive loads, the fibers of the annulus experience significant tension. As a result, the annular architecture is consistent with current remodeling theories, where the ±60° orientation of the collagen fibers, relative to the longitudinal axis of the spine, is optimally arranged to support the tensile stresses developed within a pressurized cylinder. This tissue pre-stress contributes significantly to the normal kinematics and mechanical response of the spine.
When the physical stress placed on the spine exceeds the nuclear swelling pressure, water is expressed from the disc, principally through the semipermeable cartilaginous end-plates. Consequently, significant disc water loss can occur over the course of a day due to activities of daily living. For example, the average diurnal variation in human stature is about 19 mm, which is mostly attributable to changes in disc height. This change in stature corresponds to a change of about 1.5 mm in the height of each lumbar disc. Using cadaveric spines, researchers have demonstrated that under sustained loading, intervertebral discs lose height, bulge more, and become stiffer in compression and more flexible in bending. Loss of nuclear water also dramatically affects the load distribution internal to the disc. In a healthy disc under compressive loading, compressive stress is created mainly within the nucleus pulposus, with the annulus acting primarily in tension. Studies show that, after three hours of compressive loading, there is a significant change in the pressure distribution, with the highest compressive stress occurring in the posterior annulus. Similar pressure distributions have been noted in degenerated and denucleated discs as well. This reversal in the state of annular stress, from physiologic tension due to circumferential hoop stress, to non-physiologic axial compression, is also noted in other experimental, analytic and anatomic studies, and clearly demonstrates that nuclear dehydration significantly alters stress distributions within the disc as well as its biomechanical response to loading.
The most consistent chemical change observed with degeneration is loss of proteoglycan and concomitant loss of water. This dehydration of the disc leads to loss of disc height. In addition, in humans there is an increase in the ratio of keratan sulphate to chondroitin sulphate, an increase in proteoglycan extractability, and a decrease in proteoglycan aggregation through interaction with hyaluronic acid (although the hyaluronic acid content is typically in excess of that needed for maximum aggregation). Structural studies suggest that the non-aggregable proteoglycans lack a hyaluronate binding site, presumably because of enzytruitic scission of the core protein by stromelysin, an enzyme which is thought to play a major role in extracellular matrix degeneration. These proteoglycan changes are thought to precede the morphological reorganization usually attributed to degeneration. Secondary changes in the annulus include fibrocartilage production with disorganization of the lamellar architecture and increases in type II collagen.
Currently, there are few clinical options to offer to patients suffering from these conditions. These clinical options are all empirically based and include (1) conservative therapy with physical rehabilitation and (2) surgical intervention with possible disc removal and spinal fusion. In contrast to other joints, such as the hip and knee, very few methods of repair with restoration of function are not available for the spine.
Therefore, there is a need for a minimally invasive treatment for degenerated discs which can repair and regenerate the disc. The present invention satisfies that need, as well as others, and overcomes the deficiencies associated with conventional implants and treatment methods.